U.S. patent application number 10/757314 was filed with the patent office on 2004-09-16 for ltcc based electronically tunable multilayer microstrip-stripline combline filter.
Invention is credited to Kang, Qinghua, Rahman, Mohammed Mahbubur, Shamsaifar, Khosro.
Application Number | 20040178867 10/757314 |
Document ID | / |
Family ID | 32965492 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040178867 |
Kind Code |
A1 |
Rahman, Mohammed Mahbubur ;
et al. |
September 16, 2004 |
LTCC based electronically tunable multilayer microstrip-stripline
combline filter
Abstract
A voltage-controlled tunable multilayer filter which comprises a
first resonator on a first layer of dielectric material or
low-temperature-co fired-ceramic; a second resonator coupled to
said first resonator on a second layer of dielectric material or
low-temperature-co fired-ceramic; a third resonator located on a
third layer of dielectric material or low-temperature-co
fired-ceramic coupled to said second resonator and cross coupled to
said first resonator; an input transmission line connected to said
first resonator; an output transmission line connected with said
third resonator; and a variable capacitor in at least one of said
resonators. The variable capacitor can comprise a substrate having
a low dielectric constant with planar surfaces; a tunable
dielectric film on said substrate comprising a low loss tunable
dielectric material; a metal electrode with predetermined length,
width, and gap distance; and a low loss isolation material used to
isolate an outer bias metallic contact and a metallic electrode on
the tunable dielectric. This allows the center frequency of the
filter to be tuned by changing the variable capacitor capacitance
by changing the voltage.
Inventors: |
Rahman, Mohammed Mahbubur;
(Glastonbury, CT) ; Kang, Qinghua; (Newark,
DE) ; Shamsaifar, Khosro; (Ellicott city,
MD) |
Correspondence
Address: |
James S. Finn
#2825
8650 Southwestern Blvd.
Dallas
TX
75206-2688
US
|
Family ID: |
32965492 |
Appl. No.: |
10/757314 |
Filed: |
January 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60445351 |
Feb 5, 2003 |
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Current U.S.
Class: |
333/205 |
Current CPC
Class: |
H01P 1/20336
20130101 |
Class at
Publication: |
333/205 |
International
Class: |
H01P 001/203 |
Claims
What is claimed is:
1. A voltage-controlled tunable multilayer filter comprising: a
first resonator on a first layer of dielectric material or
low-temperature-co fired-ceramic; a second resonator coupled to
said first resonator on a second layer of dielectric material or
low-temperature-co fired-ceramic; a third resonator located on a
third layer of dielectric material or low-temperature-co
fired-ceramic coupled to said second resonator and cross coupled to
said first resonator; an input transmission line connected to said
first resonator; an output transmission line connected with said
third resonator; and a variable capacitor in at least one of said
resonators.
2. The voltage-controlled tunable multilayer filter of claim 1,
further comprising a dc blocking capacitor in at least one of said
resonators.
3. The voltage-controlled tunable multilayer filter of claim 2,
further comprising DC biasing circuit associated with said
filter.
4. The voltage-controlled tunable multilayer filter of claim 3,
wherein said DC biasing lines include at least one resister to
prevent leakage into said DC biasing lines.
5. The voltage-controlled tunable multilayer filter of claim 1,
wherein there are a total of nine layers of LTCC tape or dielectric
material.
6. The voltage-controlled tunable multilayer filter of claim 5,
wherein at least two of said nine layerers are used as the inner
ground plane to implement the stripline structure.
7. The voltage-controlled tunable multilayer filter of claim 6,
wherein layer 2 and layer 6 are used as the inner ground plane to
implement the stripline structure.
8. The voltage-controlled tunable multilayer filter of claim 7,
wherein the portion of each combline resonator between said layer 2
and layer 6 is in stripline form and the remainder of the
resonators are on the top layer and in microstripline form.
9. The voltage-controlled tunable multilayer filter of claim 4,
wherein said at least one resister in the biasing circuit is
implemented in layer 1 with resistive paste.
10. The voltage-controlled tunable multilayer filter of claim 7,
wherein the input output lines are taken to the bottom plane
through the apertures in layer 2.
11. The tunable filter of claim 1, wherein said variable capacitor
comprises: a substrate having a low dielectric constant with planar
surfaces; a tunable dielectric film on said substrate comprising a
low loss tunable dielectric material; a metal electrode with
predetermined length, width, and gap distance; and a low loss
isolation material used to isolate an outer bias metallic contact
and a metallic electrode on the tunable dielectric.
12. The voltage-controlled tunable multilayer filter of claim 1,
wherein the center frequency of the filter is tuned by changing the
variable capacitor capacitance by changing the voltage.
13. A method of using voltage to tune a multilayer filter,
comprising the steps of: providing a first resonator on a first
layer of dielectric material or low-temperature-co fired-ceramic;
providing a second resonator coupled to said first resonator on a
second layer of dielectric material or low-temperature-co
fired-ceramic; providing a third resonator located on a third layer
of dielectric material or low-temperature-co fired-ceramic coupled
to said second resonator and cross coupled to said first resonator;
inputting a transmission line connected to said first resonator;
outputting a transmission line connected with said third resonator;
and varying the capacitance in at least one of said resonators.
14. The method of using voltage to tune a multilayer filter of
claim 13, further comprising the steps of including a dc blocking
capacitor in at least one of said resonators.
15. The method of using voltage to tune a multilayer filter of
claim 14, further comprising biasing said filter with a DC biasing
circuit.
16. The method of using voltage to tune a multilayer filter of
claim 15, wherein said DC biasing lines include at least one
resister to prevent leakage into said DC biasing lines.
17. The method of using voltage to tune a multilayer filter of
claim 13, wherein there are a total of nine layers of LTCC tape or
dielectric material.
18. The method of using voltage to tune a multilayer filter of
claim 17, wherein at least two of said nine layerers are used as
the inner ground plane to implement the stripline structure.
19. The method of using voltage to tune a multilayer filter of
claim 18, wherein layer 2 and layer 6 are used as the inner ground
plane to implement the stripline structure.
20. The method of using voltage to tune a multilayer filter of
claim 19, wherein the portion of each combline resonator between
said layer 2 and layer 6 is in stripline form and the remainder of
the resonators are on the top layer and in microstripline form.
21. The method of using voltage to tune a multilayer filter of
claim 16, wherein said at least one resister in the biasing circuit
is implemented in layer 1 with resistive paste.
22. The method of using voltage to tune a multilayer filter of
claim 19, wherein the input output lines are taken to the bottom
plane through the apertures in layer 2.
23. The method of using voltage to tune a multilayer filter of
claim 13, wherein said variable capacitor comprises: a substrate
having a low dielectric constant with planar surfaces; a tunable
dielectric film on said substrate comprising a low loss tunable
dielectric material; a metal electrode with predetermined length,
width, and gap distance; and a low loss isolation material used to
isolate an outer bias metallic contact and a metallic electrode on
the tunable dielectric.
24. The method of using voltage to tune a multilayer filter of
claim 13, wherein the center frequency of the filter is tuned by
changing the variable capacitor capacitance by changing the
voltage.
25. A voltage-controlled tunable multilayer filter comprising: a
first resonator on a first layer of dielectric material or
low-temperature-co fired-ceramic; a second resonator coupled to
said first resonator on a second layer of dielectric material or
low-temperature-co fired-ceramic; a third resonator located on a
third layer of dielectric material or low-temperature-co
fired-ceramic coupled to said second resonator and cross coupled to
said first resonator; an input transmission line connected to said
first resonator; an output transmission line connected with said
third resonator; and a MEMS based varactor in at least one of said
resonators.
26. The voltage-controlled tunable multilayer filter of claim 25,
wherein said MEMS varactor uses a parallel plate topology.
27. The voltage-controlled tunable multilayer filter of claim 25,
wherein said MEMS varactor uses an interdigital topology.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional Patent
Application Serial No. 60/445,351, "LTCC BASED ELECTRONICALLY
TUNABLE MULTILAYER MICROSTRIP-STRIPLINE COMBLINE FILTER" filed Feb.
6, 2003, by Mohammed Mahbubur Rahman et al.
BACKGROUND OF THE INVENTION
[0002] The present invention generally relates to tunable filters,
tunable dielectric capacitors, and, more particularly, this
invention relates to a voltage-controlled LTCC based tunable
filter.
[0003] Electronically tunable microwave filters have found wide
applications in microwave systems. Compared to mechanically and
magnetically tunable filters, electronically tunable filters have
the most important advantage of fast tuning capability over a wide
band application. Because of this advantage, they can be used in
applications such as cellular, PCS (personal communication system),
Point to Point, Point to multipoint, LMDS (local multipoint
distribution service), frequency hopping, satellite communication,
and radar systems. Electronically tunable filters can be divided
into two types: one is a dielectric capacitor based tunable filter
and the other is semiconductor varactor based tunable filter.
Compared to the semiconductor varactor based tunable filters,
tunable dielectric capacitor based tunable filters have the merits
of lower loss, higher power-handling, and higher IP3, specifically
at higher frequencies.
[0004] Tunable filters have been developed for radio frequency (RF)
applications. They are tuned electronically by using either
dielectric varactors or Micro-electro-mechanical systems (MEMS)
based varactors. Tunable filters offer service providers
flexibility and scalability, which were never possible before. A
single tunable filter solution enables radio manufacturers to
replace several fixed filters covering adjacent frequencies. This
versatility provides front-end RF tunability in real time
applications and decreases deployment and maintenance costs through
software controls and reduced component count. Also, fixed filters
need to be wide band so that the total number of filters to cover a
desired frequency range does not exceed reasonable numbers. Tunable
filters, however, are narrow band and maybe tuned in the field by
remote command. Additionally, narrowband filters at the front end
are superior from the systems point of view, because they provide
better selectivity and help reduce interference from nearby
transmitters. Two of such filters can be combined in diplexer or
duplexer configurations.
[0005] Inherent in every tunable filter is the ability to rapidly
tune the response using high-impedance control lines. The assignee
of the present invention has developed and patented tunable filter
technology such as the tunable filter set forth in U.S. Pat. No.
6,525,630 entitled, "Microstrip tunable filters tuned by dielectric
varactors", issued Feb. 25, 2003 by Zhu et al. This patent is
incorporated in by reference. Also, patent application Ser. No.
09/457,943, entitled, "ELECTRICALLY TUNABLE FILTERS WITH DIELECTRIC
VARACTORS" filed Dec. 9, 1999, by Louise C. Sengupta et al. This
application is incorporated in by reference.
[0006] The assignee of the present invention and in the patent and
patent application incorporated by reference has developed the
materials technology that enables these tuning properties, as well
as, high Q values resulting low losses and extremely high IP3
characteristics, even at high frequencies. The elaboration of the
novel tunable material technology is elaborated on in the patent
and patent application incorporated in by reference.
[0007] Also, tunable filters based on MEMS technology can be used
for these applications. They use different bias voltages to vary
the electrostatic force between two parallel plates of the varactor
and hence change its capacitance value. They show lower Q than
dielectric varactors, but can be used successfully for low
frequency applications.
[0008] Thus, there is a strong need in the communications industry
to provide several layers of dielectric material or
low-temperature-co fired-ceramic (LTCC) tape based electronically
tunable multilayer microstrip-stripline combline filter operable
over a wide frequency band and that is small in size.
SUMMARY OF THE INVENTION
[0009] The present invention provides a voltage-controlled tunable
multilayer filter which comprises a first resonator on a first
layer of dielectric material or low-temperature-co fired-ceramic; a
second resonator coupled to said first resonator on a second layer
of dielectric material or low-temperature-co fired-ceramic; a third
resonator located on a third layer of dielectric material or
low-temperature-co fired-ceramic coupled to said second resonator
and cross coupled to said first resonator; an input transmission
line connected to said first resonator; an output transmission line
connected with said third resonator; and a variable capacitor in at
least one of said resonators. The variable capacitor can comprise a
substrate having a low dielectric constant with planar surfaces; a
tunable dielectric film on said substrate comprising a low loss
tunable dielectric material; a metal electrode with predetermined
length, width, and gap distance; and a low loss isolation material
used to isolate an outer bias metallic contact and a metallic
electrode on the tunable dielectric. This allows the center
frequency of the filter to be tuned by changing the variable
capacitor capacitance by changing the voltage.
[0010] Additionally, the voltage-controlled tunable multilayer
filter can include a dc blocking capacitor in at least one of said
resonators with a DC biasing circuit associated with said filter
and the DC biasing lines can include at least one resister to
prevent leakage into said DC biasing lines.
[0011] The present invention further provides a method of using
voltage to tune a multilayer filter. This method comprises the
steps of providing a first resonator on a first layer of dielectric
material or low-temperature-co fired-ceramic; providing a second
resonator coupled to said first resonator on a second layer of
dielectric material or low-temperature-co fired-ceramic; providing
a third resonator located on a third layer of dielectric material
or low-temperature-co fired-ceramic coupled to said second
resonator and cross coupled to said first resonator; inputting a
transmission line connected to said first resonator; outputting a
transmission line connected with said third resonator; and varying
the capacitance in at least one of said resonators. The variable
capacitor used in this method can comprise a substrate having a low
dielectric constant with planar surfaces; a tunable dielectric film
on said substrate comprising a low loss tunable dielectric
material; a metal electrode with predetermined length, width, and
gap distance; and a low loss isolation material used to isolate an
outer bias metallic contact and a metallic electrode on the tunable
dielectric. The center frequency of the filter of the present
method therefore can be tuned by changing the variable capacitor
capacitance by changing the voltage.
[0012] The method can further include the step of including a dc
blocking capacitor in at least one of said resonators, thus
enabling biasing said filter with a DC biasing circuit. The DC
biasing lines can include at least one resister to prevent leakage
into said DC biasing lines.
[0013] Another embodiment of the present invention which includes a
MEMs varactor provides a voltage-controlled tunable multilayer
filter which comprises a first resonator on a first layer of
dielectric material or low-temperature-co fired-ceramic; a second
resonator coupled to said first resonator on a second layer of
dielectric material or low-temperature-co fired-ceramic; a third
resonator located on a third layer of dielectric material or
low-temperature-co fired-ceramic coupled to said second resonator
and cross coupled to said first resonator; an input transmission
line connected to said first resonator; an output transmission line
connected with said third resonator; and a MEMS based varactor in
at least one of said resonators. Further, the MEMS varactor can use
a parallel plate or interdigital topology.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 illustrates a layout of the multilayer filter of the
present invention;
[0015] FIG. 2 depicts a top layer (layer 9 in a preferred
embodiment) of the combline filter of the present invention;
[0016] FIG. 3 depicts Layer 6 of a preferred embodiment (top ground
pane for stripline) of the combline filter of the present
invention;
[0017] FIG. 4 illustrates Layer 4 of a preferred embodiment of the
combline filter of the present invention;
[0018] FIG. 5 illustrates Layer 3 of a preferred embodiment of the
combline filter of the present invention;
[0019] FIG. 6 illustrates Layer 2 of a preferred embodiment (bottom
ground pane for stripline) of the combline filter of the present
invention;
[0020] FIG. 7 illustrates Layer 1 (including resistor layer) of a
preferred embodiment of the combline filter of the present
invention;
[0021] FIG. 8 illustrates the Bottom Layer of the combline filter
of the present invention;
[0022] FIG. 9 graphically depicts the response of the filter tuned
at three different frequencies.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] It is an object of the present invention to provide a
voltage-tuned filter having a very small size, low insertion loss,
fast tuning speed, high power-handling capability, high IP3 and low
cost in the RF and microwave frequency range. Compared to
voltage-controlled semiconductor varactors, voltage-controlled
tunable dielectric capacitors have higher Q factors, higher
power-handling capability and higher third order intercept point
(IP3). Voltage-controlled tunable diode varactors or voltage
controlled MEMS varactors can also be employed in the filter
structure to achieve the goal of this object, although with
decreased performance. Yet another object of the present invention
is to have a compact filter capable of being tuned over the three
transmit bands of a wireless handset application.
[0024] A first embodiment of the present invention provides for a
tunable filter in a low-temperature-co fired-ceramic (LTCC)
package. The tuning elements are preferably voltage-controlled
tunable dielectric capacitors or, in a less preferred alternate
embodiment, MEMS varactors placed on the resonator lines of each
filter. Since the tunable dielectric capacitors show high Q, high
IP3 (low inter-modulation distortion) and low cost, the tunable
filter in the present invention has the advantage of low insertion
loss, fast tuning speed, and high power handling. It is also
low-cost and provides fast tuning. The present technology makes
tunable filters very promising in the contemporary communication
system applications.
[0025] The tunable dielectric capacitor in the present invention is
made from a low loss tunable dielectric film. The range of Q-factor
of the tunable dielectric capacitor is between 50, for very high
tuning material, and 300, for low tuning materials. It decreases
with the increase of the frequency, but even at higher frequencies,
say 30 GHz, can have values as high as 100. A wide range of
capacitance of the tunable dielectric capacitors is available; for
example, 0.1 pF to several pF. The tunable dielectric capacitor is
a packaged two-port component, in which the tunable dielectric can
be voltage-controlled. The tunable film is deposited on a
substrate, such as MgO, LaAIO3, sapphire, Aha3 and other dielectric
substrates. An applied voltage produces an electric field across
the tunable dielectric, which produces an overall change in the
capacitance of the tunable dielectric capacitor.
[0026] The tunable capacitors based on MEMS technology can also be
used in the tunable filter and are part of this invention. At least
two varactor topologies can be used, parallel plate and
interdigital. In the parallel plate structure, one of the plates is
suspended at a distance from the other plate by suspension springs.
This distance can vary in response to electrostatic force between
two parallel plates induced by applied bias voltage. In the
interdigital configuration, the effective area of the capacitor is
varied by moving the fingers comprising the capacitor in and out
and changing its capacitance value. MEMS varactors have lower Q
than their dielectric counterpart, especially at higher
frequencies, but can be used in low frequency applications.
[0027] The tunable filter with asymmetric response consists of
combline resonators implemented in microstrip-stripline form. In a
preferred embodiment it can be a 3-pole tunable combline filter as
described below. Variations of the capacitance of the tunable
capacitor affect the distribution of the electric field in the
filter, which in turn varies the resonant frequency.
[0028] The combline resonators are implemented in stripline as well
as microstrip line form. The filter needs several layers of
dielectric material or low-temperature-co fired-ceramic (LTCC)
tape. In one preferred embodiment, a three-pole filter is realized
using LTCC tapes, although it is understood that design choice
would dictate the number of poles and number of layers provided and
it is understood that any number of poles or layers are included in
the scope of the present invention. The present preferred
embodiment of the present invention and the one described below is
a filter that uses a total of nine tape layers.
[0029] Turning now to FIG. 1, the layout of the filter is shown
with all layers. All the layers have been shown separately in FIGS.
2 through 8 to assist in the understanding of the present
invention. Shown generally in FIG. 1 at 100 is the multilayer
microstrip-stripline combline filter module. Vias 105 are located
on ground planes 110 (the present invention may have several layers
of ground planes, but for purposes of the perspective of FIG. 1
will generally be referred to as 110) to connect the internal
ground planes 110 which further include opening grids 115, 145,
155, 160, 177, 195 and 197. A thruhole 120 is positioned on ground
plane 110 for the left-side microstrip-stripline resonator. A DC
bias port is provided in the bottom layer and another thruhole 140
is provided for the right-side microstrip-stripline resonator. An
isolation in the bottom layer of ground plane 110 for DC bias port
is provided at 150. A thruhole for RF/IO port is depicted at 165
and a thruhole for right-side DC bais via is shown at 170 and the
RF portion of the bottom layer at 175.
[0030] At 179 is an isolation in the bottom layer 110 for RF/IP
port 165 and at 180 is a via connecting an inner stripline to the
bottom ground plane 110. At 181 are thruholes for the left-side DC
bias via and at 183 is an inner stripline portion of the
microstrip-stripline resonator. Thruholes for center DC bias via is
provided at 185. At 190 is a via connecting upper microstrip to
upper internal ground plane 110 and at 199 is a via connecting
inner stripline to bottom ground plane 110.
[0031] Turning now to FIG. 2 is shown a top layer (layer 9) of the
combline filter of the present invention. Here, a microstrip
portion of the multilayer microstrip-stripline combline filter
module is shown at 200 with part of left-side micrstrip line shown
as 205, part of center microstrip line shown at 210 and part of
right-side of microstrip line shown at 215. A connection for
left-side resonator DC bias is shown at 220, a connection for
center resonator DC bias at 225 and a connection for right-side
resonator DC bias at 230. At 235 is part of the left-side
microstrip line for mounting a varactor; at 240 is part of center
microstrip line for mounting a varactor; and at 245 is part of
right-side microstrip line for mounting a varactor.
[0032] Referring now to FIG. 3 is depicted Layer 6 (top ground
plane for stripline) of the combline filter of the present
invention. Here upper internal ground plane of the multilayer
microstrip-stripline combline filter module is depicted at 300 and
opening grids in the ground plane are shown at 305 and 325.
Thruholes are shown at 310, 315, 320, 330, 335 and 340.
[0033] Referring now to FIG. 4 is illustrated Layer 4 of the
combline filter of the present invention. Herein, stripline portion
of left- and right-side resonators and the lower internal ground
plane are depicted at 400. Stripline portion of left-side
microstrip stripline resonator is at 405 and stripline portion of
right-side microstrip-stripline resonator is shown at 410. Tapped
RF I/O port at left-side microstrip-stripline resonator is depicted
at 420 and Tapped RF I/O port at right-side microstrip-stripline
resonator is depicted at 425.
[0034] FIG. 5 illustrates Layer 3 of the combline filter of the
present invention, wherein Asymmetrical stripline portion of center
resonator and the lower internal ground plane is shown at 500 and
lower internal ground plane for the striplines is illustrated at
505. Asymmetrical stripline portion of the center
microstrip-stripline resonator is shown at 510.
[0035] FIG. 6 illustrates Layer 2 (bottom ground pane for
stripline) of the combline filter of the present invention, wherein
lower internal ground plane of the multilayer microstrip-stripline
combline filter module is generally shown as 600 and lower internal
ground plane made from metallization is illustrated as 605.
Further, opening grid in the lower internal ground plane is at 610
and thruhole for RF I/O port via 615, thruholes for DC bias vias at
620, 625 and 630. Finally, thruhole for RF I/O port via is shown at
635.
[0036] FIG. 7 illustrates Layer 1 (including resistor layer) of the
combline filter of the present invention. The following table
illustrates the components of FIG. 7 and are connected as shown
graphically.
1 700 RF choke resistors and the bottom metallization layer 705
Bottom metallized ground plane 710 Metal catch pad for connection
to the DC bias port 715 Metal catch pad for connection to the DC
bias port 720 Metal strip for the DC bias connection 725 Metal
strip for the DC bias connection 730 Metal termination pad for the
resistor 735 RF choke resistor 740 Metal termination pad for the
resistor 745 Metal termination pad for the resistor 750 RF choke
resistor 755 Metal termination pad for the resistor 760 RF choke
resistor 765 Metal strip for the DC bias connection 770 Metal
termination pad for the resistor 775 Metal termination pad for the
resistor
[0037] FIG. 8 illustrates the Bottom Layer of the combline filter
of the present invention with RF, DC ports and the bottom
metallization layer depicted generally as 800 and bottom metallized
ground plane shown as 805. Further, DC bias ports 810 and 815 are
illustrated as well as RF I/O ports 820 and 825
[0038] The regular combline resonator is roughly one eighth of a
wavelength. If the combline resonator is implemented in one layer,
the filter size is generally large. Therefore, the comb line
resonators in the present invention are implemented in multilayer
topology to miniaturize the filter. To achieve better Q from the
resonator structure, the good portion of the resonator has been
implemented in the stripline form. The stripline portions of the
resonators are shown in FIGS. 4 and 5 as described above. The
stripline portions of the two end resonators are in the same layer
(layer 4). As shown in FIG. 5 at 500 the center resonator 510 is in
a different layer 505. The resonators are placed in different
layers to achieve less coupling between the adjacent resonators and
to achieve the desired cross coupling between the two end
resonators. The cross coupling between the two end resonators helps
to create a transmission zero on the high side of the passband of
the filter. This improves the high side selectivity at the expense
of the low side selectivity degradation. This is desired for the
transmit filters in the handset application.
[0039] The striplines go though apertures in the top ground plane
(layer 6) to the top layer of the board. The microstrip portions of
the resonators are folded back as shown in FIG. 1. Therefore, the
size of the filter is reduced by almost half. Microstrip portions
of the resonators are used to mount the tuning components
(dielectric varactors/MEM varactors/varactor diode) and the DC
blocking capacitors. The combline resonators are shorted to both
ends. Therefore, the DC blocking capacitors are necessary to apply
voltage to the varactors for tuning. The DC biasing circuit is
implemented by a short length of high impedance line and a high
resistor. It is possible to use a conventional quarter wave length
high impedance line with quarter wave length radial stub for the
biasing circuit; however, it occupies a larger amount of space,
which makes the tunable filter larger. The varactors of the present
invention have the good characteristic of drawing current in the
few uA range. Therefore, the resistor in the biasing line doesn't
drop any appreciable voltage. The assignee of the present invention
has developed the varactors that can be used in the present
invention. One such example is provided in co-owned in U.S. Pat.
No. 6,531,936, entitled, "Voltage tunable varactors and tunable
devices including such varactors" filed Mar. 11, 2003, by Chiu et
al. This patent is incorporated in by reference.
[0040] Turning now to FIG. 9, the graph illustrates the desirable
performance abilities and frequency filter flexibility provided by
the present invention by graphically depicting the response of the
filter tuned at three different frequencies. Shown on the graph at
900 is the typical filter performance for the tunable multilayer
microstrip-stripline combline filter. 905 illustrates the filter
response of S-parameters in dB at 905 and tunable filter frequency
range in GHz at 910. The graph further shows the filter response
when varactors are at low or zero bias voltage at 915 and filter
response when varactors are at an intermediate bias voltage 920 and
filter response when varactors are at high bias voltage 925.
[0041] While the present invention has been described in terms of
what are at present believed to be its preferred embodiments, those
skilled in the art will recognize that various modifications to the
disclose embodiments can be made without departing from the scope
of the invention as defined by the following claims.
* * * * *